Columns and Stationary Phases

Different support materials have been applied in HTCGC. Leached and deactivated borosilicate glass provides an excellent surface for high temperature

Figure 1 Thermogravimetric analysis of triolein. Ramp from 20 to 500°C at 5°C min"1. (A) pure nitrogen, (B) air.

work. The surface can be coated with different high temperature phases for applications up to 450°C. Glass columns are, however, not really accepted for routine work as they break easily and are difficult to handle. Nevertheless, some laboratories still use glass columns because of their excellent performance for specific applications (Blum and Aichholz). Leached and persilylated fused silica is nowadays mostly applied. The outer polyimide coating of classical fused silica open tubular (FSOT) columns withstands temperatures up to 400°C, thus covering most of the applications of HTCGC. To increase the lifetime of the columns, they are often wrapped in aluminium foil to avoid contact with oxygen which initiates polyimide decomposition. Aluminium-clad fused silica columns have been introduced for applications up to 450°C. Because of the different expansion coefficients of fused silica and aluminium, the columns can become brittle under continuous heating and cooling conditions. An excellent alternative to glass, polyim-ide-coated fused silica and aluminium-clad fused silica columns are the recently introduced metal columns for which the active surface has been passivated, for example with a thin layer of fused silica. Silcosteel and Ultimetal capillary columns for high temperature work are commercially available. The suppliers both make special columns to perform simulated distillation by GC according to ASTM methodD 2887.

A large number of stationary phases have been synthesized for HTCGC. The group of W. Blum has been very active in this respect. Their HTCGC experiments, including the synthesis of the phases and the coating of capillary columns, are summarized in an excellent book. The three phases most often applied in HTCGC are methylsilicone, diphenyldimethyl-silicone with phenyl contents varying between 5% and 65% and a carborane-modified methylsilicone. The main reason for this is that high temperature columns with these phases are commercially available. The phases are OH-terminated and immobilization by polycondensation takes place at high temperatures after coating. This increases thermal stability and makes the columns solvent resistant.

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